Type B2 Thymoma

Thymoma with prominent large epithelial cells with a roughly even admixture of numerous lymphocytes

Prominent large epithelial cells with numerous admixed lymphocytes
- Lacks extensive areas of virtually pure epithelial cells
- Lacks extensive areas of virtually pure lymphocytes
Prominent polygonal epithelial cell population
- Frequent aggregates of epithelial cells
- Lack long thin processes
- Nuclei vesicular with prominent nucleoli
- May palisade
Prominent lymphoid infiltrate
- Lymphocytes may outnumber epithelial cells but do not obscure them
Perivascular spaces common
Medullary foci and well formed Hassall corpuscles are absent to infrequent

Epithelial cells
- Keratin positive
- EMA negative
- CD20 negative (we have seen rare positive cells)
- CD5 negative
Lymphocytes
- Immature T phenotype (CD1a, CD99, TdT)
- T cells express most pan-T cell markers (CD3, CD5)
- Few B cells

Associations
- Myasthenia gravis
- Pure red cell aplasia
- Hypogammaglobulinemia
Usually presents at low stage (75% of cases)
- May be aggressive
Widely variable long term survival depending on stage

Type B1 Thymoma	Type B2 Thymoma
Predominantly lymphocytes with inconspicuous epithelial cells	Both lymphocytes and epithelial cell prominent
Numerous scattered pale medullary foci	Only occasional medullary foci

Type B2 Thymoma	Type B3 Thymoma
Both lymphocytes and epithelial cells are easy to see	Predominantly epithelial with few lymphocytes
Lacks any epidermoid features	Frequently has epidermoid differentiation (but lacks overt keratinization and bridges)
Epithelial CD5 negative	Epithelial cells may be focally positive for CD5
Epithelial cell nuclei are large round vesicular with prominent nucleoli	Epithelial cell nuclei moderate in size, not typically large and vesicular

Type AB Thymoma	Type B2 Thymoma
Epithelial cells have small oval to spindled nuclei with dispersed chromatin and inconspicuous nucleoli	Epithelial cells have vesicular nuclei with prominent nucleoli
Areas of Type A are usually present	Type A areas not present

Both may be composed of an essentially even mixture of epithelial cells and lymphocytes

Type B2 Thymoma	Thymic Lymphoepithelial Carcinoma
Mild cytologic atypia at worst	Cytologically atypical
Epithelial cell mitotic figures rare	Epithelial cell mitotic figures may be frequent
Lymphocytes are immature T cells	Lymphocytes are mature T and B cells

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